Heating resistor type air flow [rate] measuring apparatus

ABSTRACT

A heating resistor type air flow rate measuring apparatus is provided with a couple of heating resistors placed at the positions where those resistors may interfere thermally each other with respect to an air flow, and a couple of driving circuits for driving those heating resistors. The air flow rate signal is obtained by calculating the difference between the output signals of a couple of heating resistors in terms of heat radiation rate effected by an air flow, and adding the difference value onto the output signal of one of heating resistors.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an air flow meter for measuringthe intake air flow rate to an internal combustion engine, especially, aheating resistor type air flow rate measuring apparatus for measuringthe air flow rate in the condition accompanying with a backward flow ina pulsating flow.

[0002] The air flow in an internal combustion engine is pulsated by acontinuous make and break operation of the intake air valve. Thepulsation is so amplified by the effect of the columnar vibration in theintake air duct , and the air flow inside the intake air pipe becomes abackward flow in a specific condition related to the number of rotationsof the engine and the aperture of the throttle valve. This backward flowbrings various bad effects in the heating resistor type air flow ratemeasuring apparatus. As for an apparatus for solving this problem, aprior art disclosed in Japanese Patent Application Laid-Open No.1-206223 (1989) is known as a air passage structure having a sub airduct shaped in a letter I (or L) which is used as a means for increasingthe accuracy of the measurement of the heating resistor type air flowrate measuring apparatus operated under a condition where a backwardflow occurs with a pulsating flow. In this prior art, the air passage isso configured that the backward flow directly blows against the heatingresistor by forming a wall facing against to the backward flow.

[0003] As for another apparatus for reducing the bad effect of thebackward flow, a prior art is disclosed in Japanese Patent ApplicationLaid-Open No. 62-812 (1987). In this prior art, similarly to the presentinvention, by detecting the direction of the air flow by using thethermal interference between a couple of heating resistors, the outputvoltage signals from the heating resistors are altered by judging thedirection of the air flow; when the air flow is a forward flow, theoutput voltage to be used is selected from the output voltage signal ofthe heating resistor for the forward flow; when the air flow is abackward flow, the output voltage signal from the heating resistor forthe backward flow is selected.

[0004] In general, it is difficult to measure the direction of the airflow, forward or backward, selectively only by using a single heatingresistor. In order to solve this problem, for example, as shown in FIG.10, in observing the average output of the heating resistor type airflow rate measuring apparatus by varying the boost pressure by makingthe throttle valve gradually open while the number of rotations of theengine is maintained to be constant, the average output of the air flowrate increases linearly as the intake negative pressure increases undera certain threshold value, and for the boost pressure above a certainthreshold value, the average output of the air flow rate is estimated tobe larger than the actual air flow rate (which is designatedover-shooting phenomena). Though the pulsation in the air flow rate inthe heating resistor type air flow rate measuring apparatus isrelatively small when the throttle valve opens with a small aperture,the amplitude of the pulsation in the air flow rate increases as thethrottle valve gets to open, and finally, at the throttle valve anglelarger than a certain angle (about 30 to 50) (in the right region of Ain FIG. 10), the pulsation amplitude contains the backward flowcomponent. Thus, when the backward flow occurs, as the heating resistorcan not discriminate the direction of the air flow whether forward orbackward as described above, the average output of the air flow rate isestimated with the forward flow component as well as the backward flowcomponent, and thus takes larger values.

[0005] By means of forming a wall against the direction of the backwardflow as described above for the prior art, and making the air passagestructure so that the backward flow may not blow directly against theheating resistor, it is possible to reduce the estimation error for theaverage output. However, the reduced error with this means is only thehalf of the overall error. This is because the amount of forward flowincreases as the amount of backward flow increases. Thus, in order toreduce the estimation error due to the backward flow, it is necessary toreduce the output value of the forward flow when the backward flowoccurs or distract the backward flow component from the forward flowcomponent as well as the measurement of the forward flow component.There is a prior art related to this solution in which, in case that thebackward flow is observed by detecting the direction of the air flow bycomparing the output signals from those two heating resistors by using acouple of heating resistors as disclosed in the other prior artdescribed above, the backward flow component is subtracted from theforward flow component. This method has yet another problem. One isrelated to the reduction of resolution in supplying data to the microcomputer. DC voltage handled by many micro computers used for generalautomotive applications is between 0 and 5.12 (V). However, in thismethod where both the forward flow and the backward flow have a similarrelationship between the air flow rate and the output voltage, theresolution of the forward flow is reduced. In an extreme case that thethreshold voltage 2.56(V) is used and that the range below 2.56(V) isfor the range of the output voltage for the backward flow and the rangeover 2.56(V) is for the range of the output voltage for the forwardflow. Thus, the resolution of the output voltage for the forward flow inthis case is half of the resolution when all the range between 0 and2.56(V) can be used for the forward flow. Though the threshold voltage2.56(V) in this case is a little extreme case, the resolution for theforward flow is reduced because the threshold voltage should bedetermined between 1(V) and 2(V) in order to measure the backward flowprecisely.

[0006] If the heating resistor has a thermal response delay, thedetection of the backward flow is delayed when comparing the outputsignals from the heating resistors, this detection delay gives an effectto the measurement precision. This can be illustrated with FIGS. 11A and11B; when the backward flow begins to rise up at the point B in FIG.11A, the output signal level of the backward flow does not exceed theoutput signal level of the forward flow, and therefore, the existence ofthe backward flow is not proved until the output signal level of thebackward flow reaches at the point C; thus, the detection of thebackward flow is so delayed.

[0007] Further, as disclosed in Japanese Patent Application Laid-OpenNo. 62-812 (1987), the conventional apparatus determines a direction ofair flow by using two heating resistors and produces an output signal byusing either one of detection signals. A noise component produced due tothe mutual interference the two heating resistors and included in theoutput signal is moderated by attenuating the alternating currentcomponent.

[0008] However, because the output signal is attenuated in the prior artapparatus, there was the problem that the delay of detection becomeslarge at the time when the air flow is inverted and thus the precisionof measurement deteriorates.

Summary of the Invention

[0009] Therefore, a first object of the present invention is to increasethe precision of the measurement of the air flow rate in the pulsatedflow accompanying with the backward flow in a practical on-boardenvironment, which is one of the major problem in the above describedheating resistor type air flow rate measuring apparatus, and to providea heating resistor type air flow rate measuring apparatus which hasadvantages in handling easiness, reliability and cost.

[0010] A second object of the present invention is to provide animproved heating resistor type air flow rate measuring apparatus whichcan reduce the above-mentioned noise, maintaining the precision ofmeasurement.

[0011] In order to solve the above-described first problem, a couple ofheating resistors are made to be place at the positions on which thoseinterfere with the air flow with respect to thermal properties, and ifthe air flow is a forward flow, the output signal from the sensor iscorrected by the electronic circuit so that the output signal for theforward flow may be equal to the output signal for the backward flow,but if the air flow is a backward flow, the difference between theoutput signal for the forward flow and the output signal for thebackward flow is so adjusted as to be larger. In addition, the largerone of the output signals from those two heating resistors is soadjusted as to be equalized to be the lower one, and if the air flow isa backward flow, by reducing the output signal of the heating resistorfor the forward flow, the overall average value of the output signals isthus so adjusted as to be lowered. In this method, the differencebetween the output signal for the forward flow and the output signal forthe backward flow which occurs only when the air flow is a backward flowis used as the correction value. With this method, the switchingoperation of the output signals for the forward flow and the backwardflow by using the switching circuit can be eliminated. And furthermore,the threshold value for separating the forward flow and the backwardflow is not required and the output voltage used for the heatingresistor type air flow rate measuring apparatus can be varied between 0and 5.12(V), and therefore, a higher resolution for the output signalcan be established when the air flow is a forward flow. As thedifference between the output signal from the heating resistor for theforward flow and the output signal from the heating resistor for thebackward flow necessarily arises when the air flow is a backward floweven if the heating resistors have a thermal response delay, thedetection and judgment of the backward flow can be performed precisely.

[0012] Further, the preferable apparatus for attaining theabove-described second object is as follows.

[0013] A heating resistor type air flow rate measuring apparatus inwhich a forward flow detection signal is detected from a heating currentnecessary to heat a forward flow heating resistor installed in an airpassage to the predetermined temperature, and a backward flow detectionsignal is detected from a heating current necessary to heat a backwardflow heating resistor installed in the air passage to the predeterminedtemperature, comprising;

[0014] a cancelling means for cancelling a differential mode noiseincluding in each of the detection signals by adding the component ofthe alternating current of the backward flow detection signal to theforward flow detection signal and adding the component of thealternating current of the forward flow detection signal to the backwardflow detection signal.

[0015] Another preferable apparatus related to the second object is asfollows.

[0016] A heating resistor type air flow rate measuring apparatusprovided with a pair of air flow rate detecting parts for detectingheating currents necessary to heat a forward and a backward flow heatingresistor installed in an air passage to the predetermined temperature,respectively, as a forward flow detection signal and a backward flowdetection signal, in order to output an air flow rate signal including adirectional component of the air flow in the air passage by using eachdetection signal, further comprising:

[0017] a cancelling means for cancelling differential mode noisesincluding in the forward and the backward detection signals by addingthe component of the alternating current of the backward flow detectionsignal to the forward flow detection signal and adding the component ofthe alternating current of the forward flow detection signal to thebackward flow detection signal, and outputting the forward and thebackward flow cancellation signals;

[0018] wherein an air flow rate signal is output by using the forwardand the backward flow cancellation signals instead of the forward andthe backward flow detection signals.

[0019] Still further preferable apparatus related to the second objectis as follows.

[0020] A heating resistor type air flow rate measuring apparatuscomprising:

[0021] a pair of air flow rate detecting parts for detecting heatingcurrents necessary to heat a forward and a backward flow heatingresistor installed in an air passage to the predetermined temperature,respectively, as a forward flow detection signal and a backward flowdetection signal, a signal comparing means for determining the directionof the air flow in the air passage by the comparison of large and smallof the forward and the backward flow detection signals,

[0022] a signal selecting means for selecting one of the forward and thebackward flow detection signals on the basis of the result ofdetermination, and

[0023] a differential amplifying circuit for switching and inputting theforward and the backward flow detection signals, adding an alternatingcurrent component of the backward flow detection signal to the inputforward flow detection signal, and switching and outputting either oneof an output signal higher than a reference voltage in proportion to theadded signal and an output signal lower than the reference voltage inproportion to the input backward flow detection signal;

[0024] wherein an air flow rate signal including a directional componentof the air flow is output by using the output signal from thedifferential amplifying circuit.

[0025] Still further preferable apparatus related to the second objectis as follows.

[0026] A heating resistor type air flow rate measuring apparatuscomprising:

[0027] a pair of air flow rate detecting parts for detecting heatingcurrents necessary to heat a forward and a backward flow heatingresistor installed in an air passage to the predetermined temperature,respectively, as a forward flow detection signal and a backward flowdetection signal,

[0028] a signal comparing means for determining the direction of the airflow in the air passage by the comparison of large and small of theforward and the backward flow detection signals,

[0029] a signal selecting means for selecting one of the forward and thebackward flow detection signals on the basis of the result ofdetermination, and

[0030] a differential amplifying circuit for switching and inputting theforward and the backward flow detection signals, inverting the phase ofan alternating current component of the forward flow detection signaland adding the resultant signal to the input backward flow detectionsignal, and switching and outputting either one of an output signalhigher than a reference voltage in proportion to the forward flowdetection signal and an output signal lower than the reference voltagein proportion to the added signal;

[0031] wherein an air flow rate signal including a directional componentof the air flow is output by using the output signal from thedifferential amplifying circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1 is a block diagram of the heating resistor type air flowrate measuring apparatus according to one embodiment related to a firstobject of the present invention.

[0033]FIG. 2 shows output signal curves of the individual heatingresistors in case of altering the air flow directions when using thethermal interference between two heating resistors.

[0034]FIG. 3 shows output signal curves of the individual heatingresistors in case of altering the air flow directions showing oneembodiment related to the first object of the present invention.

[0035]FIG. 4 is a circuit diagram of the output correction part in theheating resistor type air flow rate measuring apparatus showing oneembodiment related to the first object of the present invention.

[0036]FIG. 5 shows pulsating waveforms of the output signals of theheating resistors in the existence of the pulsated air flows in theexperiments using the heating resistor type air flow rate measuringapparatus according to one embodiment related to the first object of thepresent invention.

[0037]FIG. 6 is a block diagram of the heating resistor type air flowrate measuring apparatus according to another embodiment related to thefirst object of the present invention.

[0038]FIG. 7 is a block diagram of the system where the signalprocessing apparatus has a function of the heating resistor type airflow rate measuring apparatus showing another embodiment related to thefirst object of the present invention.

[0039]FIG. 8 is one example structure of the heating resistor type airflow rate measuring apparatus related to the first object of the presentinvention.

[0040]FIG. 9 is another example structure of the heating resistor typeair flow rate measuring apparatus related to the first object of thepresent invention.

[0041]FIG. 10 shows an over-shooting phenomena of the heating resistorsin case of altering the intake negative pressure by opening graduallythe throttle while keeping the constant number of rotations of theengine.

[0042]FIG. 11 shows output signals of the individual heating resistorsat the individual throttle angles in case of using the output signalalternation method in case of using the heating resistors havingresponse delay characteristics.

[0043]FIG. 12 shows a system controller diagram for controlling theinternal combustion engine by using the heating resistor type air flowrate measuring apparatus related to the first object of the presentinvention.

[0044]FIG. 13 is a circuit diagram showing a heating resistor type airflow rate measuring apparatus according to a first embodiment related toa second object of the present invention.

[0045]FIG. 14 is a circuit diagram showing a heating resistor type airflow rate measuring apparatus according to another embodiment related tothe second object of the present invention.

[0046]FIG. 15 is a circuit diagram showing a heating resistor type airflow rate measuring apparatus according to a third embodiment related tothe second object of the present invention.

[0047]FIG. 16 is a circuit diagram showing a heating resistor type airflow rate measuring apparatus according to a fourth embodiment relatedto the second object of the present invention.

[0048]FIG. 17 is a circuit diagram showing a heating resistor type airflow rate measuring apparatus according to a fifth embodiment related tothe second object of the present invention.

[0049]FIG. 18 is a circuit diagram showing a heating resistor type airflow rate measuring apparatus according to a sixth embodiment related tothe second object of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] Preferred embodiments of the present invention will be explainedhereinafter. Firstly, the preferred embodiments related to the apparatuswhich can attain the first object will be described with reference toFIGS. 1 to 12.

[0051]FIG. 1 is a block diagram showing the structure of the heatingresistor type air flow rate meter as one embodiment of the presentinvention. A couple of heating resistors for air flow rate measurementare installed in the intake air duct of the engine. In FIG. 1, the leftside of the intake air duct directs to the air cleaner, and the rightside directs to the engine. Therefore, the air flow running in theintake air duct from the air cleaner to the engine is defined as theforward flow air flow 4, and the air flow running in the oppositedirection is defined as the backward flow air flow A couple of heatingresistors are placed in the intake air duct, and each of which is drivenby the independent drive circuit, respectively Though a single drivecircuit can drive both of the heating resistors theoretically, therehappens a thermal response delay with this configuration in which thefrequency response for about 20 to 200 Hz can not be established in apractical engine operation environment, and hence, the direction of theair flow can not be discriminated. This drive circuit is controlled infeedback mode by supplying the heating flow to the heating resistors sothat the temperature difference between the heating resistors and theheat-sensitive resistors installed separately for measuring the intakeair temperature may be maintained to be a constant value. Those twoheating resistors are placed in the positions where the heated air flowsare interfered thermally with the heating resistors at the upper streamor the down stream of the air flow, respectively. When the forward flowair flow occurs, the heat generated by the forward flow heating resistor1 tries to heat up the backward flow heating resistors 2 located in thedown stream, and when the backward flow air flow occurs, the heatgenerated by the backward flow heating resistor 2 tries to heat up theforward flow heating resistor 1 located in the upper stream. With thisconfiguration, for example, when the forward flow occurs, the heatingflow for keeping the constant temperature difference between thebackward flow heating resistor 2 and its corresponding heat-sensitiveresistor can be less than the heating flow for the forward flow heatingresistor 1 because the backward flow heating resistor 2 gets the heatgenerated by the forward flow heating resistor 1. Thus, the comparisonbetween the heating flows of those two heating resistors can teaches thedirection of the air flow, forward flow or backward flow, and the airflow rate.

[0052]FIG. 2 shows the characteristic of the air flow measurement, wherethe air flow rate is plotted with respect to the output signal from theheat resistors extended in the horizontal axis, each curve correspondingto the cases for forward flow and backward flow, respectively. As theoutput signal of the individual heating resistors fundamentallycorresponds to the heating flow supplied to the individual heatingresistors, when the forward flow occurs, the output signal of theforward flow heating resistor is larger, and the output signal of thebackward flow heating resistor is smaller. Though the heating flow is sodefined as shown above, the relationship in terms of the output signalvoltage can be arbitrarily adjusted by the output control by the zerospan circuit composed together with the drive circuit.

[0053]FIG. 3 shows an example of the output signal characteristic of twoheating resistors used in the heating resistor type air flow ratemeasuring apparatus of the present invention. In the present invention,when the forward flow occurs, the output signal characteristic of theheating resistor for the forward flow and the output signalcharacteristic of the heating resistor for the backward flow areadjusted so as to be approximately equivalent to each other. Thus, theoutput signals from two heating resistors when the forward flow occursare identical to each other, but the difference between the outputsignals shown in FIG. 3 when the backward flow occurs is greater thanthe difference between the output signal shown in FIG. 2. This can beexplained as the followings; though the heating flow used in the caseshown in FIG. 3 is the same as the heating flow in FIG. 2, the backwardflow heating resistor gets the heat generated by the forward flowheating when the forward flow occurs, and the output signal of thebackward flow heating resistor is amplified by the zero span circuit soas to make larger the sensitivity of the backward flow heating resistorto the air flow rate when the generic output signal of the backward flowheating resistor. According to this output signal characteristics, theoutput signal can be corrected by the following formula 1 when thebackward flow occurs.

Vout=Vf−kx(Vr−Vf)+Voffset  (1)

[0054] where, Vout: the output signal of the heating resistor type airflow rate measuring apparatus after the backward flow correction,

[0055] Vf: the output signal of the forward flow heating resistor,

[0056] Vr: the output signal of the backward flow heating resistor,

[0057] k: an arbitrary constant value,

[0058] Voffset: an offset value for the output signal (defined ifnecessary),

[0059] In the above formula, the term, kx(Vr−Vf) represents thecorrection term when the backward flow occurs. As the output signals oftwo heating resistors are the same when the forward flow occurs, thecorrection term is zero, and the output signal of the forward flowheating resistor is used. On the other hand, as the output signal of thebackward flow heating resistor is higher when the backward flow occurs,the component due to the backward flow can be corrected. Andfurthermore, by adding the constant value k to the difference betweenthe output signals of the forward flow heating resistor and the backwardflow heating resistor, a flexible correction can be done. The offsetvalue for the output signal, Voffset, is defined if necessary.

[0060]FIG. 4 shows an example of the circuit structure based on theformula 1. This circuit is composed of three operational amplifiers.Those operational amplifiers have their own functions. The output V1 ofOP1 is used for supplying the difference between the output signals ofthe forward flow heating resistor and the backward flow heating resistor(the term (Vr−Vf) in the formula 1). The output V2 of OP2 represents themultiplication of the output V1 of OP1 and the constant value k definedby the ratio between R1 and R2. (kx(Vr−Vf) in the formula 1). The finaloutput Vout of OP3 represents the summation of the output OP2, theoutput signal of the forward flow heating resistor and the offset valuefor the output signal as defined by the formula 1. In FIG. 3, therectangular portion defined with a broken line is an RC filter which isaimed to enable to eliminate noises in the output signal and allow theoutput value close to the average of the output signal corresponding tothe amplitude of the pulsating flow to be read into the control unitwithout influence of the sampling timing. The RC filter may be imbeddedinto the signal input part of the engine control unit in the circuits ofthe heating resistor type air flow rate measuring apparatus. Though thiscircuit is basically composed of three operational amplifiers, it ispossible to establish the equivalent circuit composed of two operationalamplifiers by reforming the formula 1. The detail structure of theequivalent circuit is not shown here.

[0061]FIG. 5 is the observation result of the pulsated flow wave formsin the pulsated operation region accompanying a backward flow in theheating resistor type air flow rate measuring apparatus of the presentinvention mounted on the actual engine which has the circuit structureshown in FIGS. 1 and 4 and the output signal characteristic shown inFIG. 3. Two curves laying in the lower part of the chart represent theoutput signals of the forward flow heating resistor and the backwardflow heating resistor, respectively, and the solid line curve representthe output signal of the heating resistor type air flow rate measuringapparatus of the present invention. The output signal of the heatingresistor type air flow rate measuring apparatus according to the presentinvention is defined by the formula 1. For reference, the output signalgenerated by adding only the offset value to the output signal of theforward flow heating resistor is shown by the broken line. Referring tothe output signals of the forward flow heating resistor and the backwardflow heating resistor at first, the output signal of the forward flowheating resistor and the output signal of the backward flow heatingresistor are almost the same when the forward flow occurs, but theoutput signal of the backward flow heating resistor is larger than theoutput signal of the forward flow heating resistor when the backwardflow occurs. Those are output signal characteristics in accordance withthe output signal characteristics shown in FIG. 3. And furthermore,referring to the final output signals, in comparison with the outputsignal generated by adding only the offset value to the output signal ofthe forward flow heating resistor, shown by the broken line, the outputsignal corrected with the backward flow component is almost equal to theoutput signal generated by adding only the offset value to the outputsignal of the forward flow heating resistor when the forward flowoccurs, but smaller when the backward flow occurs, which means that theaveraged output signal when the backward flow occurs can be made to bereduced. Thus, it is proved experimentally that the air flow ratemeasuring apparatus of the present invention can detects the backwardflow, and that there is such an effect that the output signal of theforward flow heating resistor can be reduced when the backward flowoccurs which is the primary object of the present invention.

[0062]FIG. 6 is a block diagram of the heating resistor type air flowrate meter in another embodiment of the present invention. The basicstructure of the block diagram in FIG. 6 which is almost the same asthat in FIG. 1, and the specific difference from FIG. 1 is that a heateris placed between a couple of heating resistors so that the thermalinterference may be established between the heater and the individualheating resistors in stead of exchanging heat directly between twoheating resistors. The reason why the structure shown in FIG. 6 is usedis that, if the distance between two heating resistors is too short, thefinal output signal of the heating resistors is disturbed due to theheat exchange between the heating resistors even in thesingle-directional air flow, resultantly, leading to the noise in theoutput signal of the heating resistor type air flow rate measuringapparatus. It is evident that the heat interference between the heatresistors can not be fully established and hence, the air flow directioncan not be detected if two heat resistors are kept away from each othertoo much.

[0063]FIG. 7 is a block diagram of the heating resistor type air flowrate meter and its output signal processing apparatus in anotherembodiment of the present invention. The basic structure of the blockdiagram in FIG. 7 which is almost the same as that in FIG. 1, and thespecific difference from FIG. 1 is that the circuit of the heatingresistor type air flow rate measuring apparatus only comprises a coupleof heating resistors and the zero span circuit for their output signals,where two individual signals for the forward flow and the backward floware supplied to the signal processing apparatus, and those outputsignals are processed for the signal correction and the detection of theair flow direction by the signal process apparatus. In this embodiment,by enabling a part of the signal processing apparatus to operate as thesignal processing function of the heating resistor type air flow ratemeasuring apparatus, there is such an advantage that the circuitstructure of the heating resistor type air flow rate measuring apparatusitself can be simplified.

[0064]FIG. 8 is a schematic structure of the heating resistor type airflow rate measuring apparatus of the present invention. The structurecontains a circuit board 8 on which the zero span circuit and the signalprocessing circuit are integrated, a protector member including ahousing member 9 and a cover member 10 for protecting the circuit board8, sensor members including the heating resistors and the sensingresistors, conductive members 11 for connecting electrically between thesensor members and the circuit board, a supporting member for supportingthe sensor members and the connecting members, a sub air route in whichthe heating resistors are placed, and a connector part 14 as interfacesto the outside of the apparatus, and all of those members are configuredas a single module. With the sensor part of the module and the sub airpassage and other members inserted in the penetration hole 16 of thebody member 15 in which installed is a honeycomb lattice 17 for reducingthe disturbance in the air flow comprising the main air passage in theintake air passage of the combustion engine, the overall structure ofthe heating resistor type air flow rate measuring apparatus is so formedby fixing the module and the body with screws.

[0065]FIG. 9 is the structure of the apparatus where the body membercomprising the main air passage of the intake air passage is not formedas a part of the heating resistor type air flow rate measuringapparatus, but is configured by using the intake air passage compositeduct of the combustion engine. In this embodiment, what is used as thebody member is the composite member of the air cleaner used for removingthe dust in the air supplied into the engine. The body member is placedin the air flow at the down stream of the air cleaner element 22. By apenetration hole 16 is formed in the air cleaner housing compositionmember formed with the duct 23 used as the main air passage of theheating resistor type air flow rate measuring apparatus as a singleunit, and thus, the single unit containing the heating resistor type airflow rate measuring apparatus and the air cleaner housing compositionmember is fixed with screws. According to this structure, by using theexisting components, a heating resistor type air flow rate measuringapparatus without a newly-built body can be provided at low cost.

[0066] Finally, referring to FIG. 7, another embodiment of the presentinvention applied to the internal combustion engine using an electronicfuel injection system.

[0067] The intake air 37 coming through the air cleaner 24 goes throughthe body of the heating resistor type air flow rate measuring apparatus1, the intake duct 25, the throttle body 28 and the intake manifold 29having the injector 30 for injecting the fuel, and reaches the enginecylinder 32. The gas generated in the engine cylinder 33 is exhaustedthrough the exhaust manifold 34.

[0068] The air flow rate signal supplied by the circuit module of theheating resistor type air flow rate measuring apparatus, the throttlevalve angle signal supplied by the throttle angle sensor 27, the oxygencontent signal supplied by the oxygen content sensor installed in theexhaust manifold 34 and the engine rotation velocity signal supplied bythe engine tachometer 31 are put into the control unit 36, where anoptimal fuel injection amount and an optimal valve aperture arecalculated and determined by using those signals, and the injector 30 adthe idol control valve 26 are controlled by using those calculatedoptimal values.

[0069] Nextly, the preferred embodiments related to the apparatus whichcan attain the second object of the present invention will be describedin detail with reference to FIGS. 13 to 18.

[0070]FIG. 13 is a circuit diagram showing a heating resistor type airflow rate measuring apparatus according to one embodiment of the presentinvention. The heating resistor type air flow rate measuring apparatusof the present embodiment is constructed by using air flow ratedetecting parts 48, 58, characteristic adjusting circuits 61, 62, acancelling means 90, a switching means 51, and a differential amplifier71.

[0071] In FIG. 13, the air flow rate detecting part 48, 58 are providedwith Wheatstone bridge circuits comprising heating resistors 41, 51, airtemperature detecting resistors 42, 52 and resistors 43, 44, 45, 53, 54,55, and constant temperature control circuits comprising operationalamplifiers 46, 56 and transistors 47, 57 which supply a current to theWheatstone bridge circuits and correct the resistance values of theheating resistors 41, 51 to be constant, in accordance with the value ofthe air temperature detecting resistors 42, 52. The pair of air flowrate detecting part 48 and 58 detect heating currents for the heatingresistors 41 and 51 by using the resistors 43 and 53 for detecting thecurrents, respectively, and output a forward flow detection signal V2Fand a backward flow detection signal V2R, respectively. The heatingresistors 41 and 51 each has higher detection sensitivity with respectto either one of the forward flow and the backward flow.

[0072] In other words, one air flow rate detecting part 48 detects theheating current necessary to heat the forward flow heating resistor 41positioned in an air passage to the predetermined temperature as theforward flow detection signal V2F, and the other air flow rate detectingpart 58 detects the heating current necessary to heat the backward flowheating resistor 51 installed in an air passage to the predeterminedtemperature as the backward flow detection signal V2R.

[0073] The forward flow detection signal V2F and the backward flowdetection signal V2R are adjusted in characteristic by characteristicadjusting circuits 61 and 62, respectively, and are output as a forwardflow adjustment signal VOF and a backward flow adjustment signal VOR,respectively. After then, these adjustment signals VOF and VOR arecompared with each other by a comparater 69, and the larger signal isselected by a switching circuit 70. Namely, a direction of air flow isdetermined by the comparison of large and small of the forward flowadjustment signal VOF and the backward flow adjustment signal VOR in thecomparater 69 or signal comparing means.

[0074] Finally, a differential amplifier 71 inverses the voltage whenthe backward flow was detected, amplifies a forward flow cancellationsignal VOUTF or a backward flow cancellation signal VOUTR, which is anoutput signal VOUT, and outputs “flow rate signal VG including thecomponent of the direction of air flow”If the heating resistor 1 fordetecting the forward flow and the heating resistor 11 for detecting thebackward flow are thermally connected to each other, then the change inthe heating current of each of the heating resistors has thecomplimentary relationship. Further, the forward flow adjustment signalVOF and the backward flow adjustment signal VOR have differential modenoises of which phases is inverted with each other. Similarly to theprior art, if either one of these adjustment signals including thedifferential mode noises is selected and used as an output signal VOUTof the flow meter, then the output signal VOUT and the flow rate signalVG also include the noises. While the component of the differential modenoise can be eliminated by using a method of attenuating, for example,averaging the components of alternating currents of the forward andbackward flow adjustment signals, there is the problem in which thedelay of detection becomes large due to the attenuation when the airflow was inverted.

[0075] Accordingly, in the present invention, the noise can beeliminated without the delay of detection at the time when the air flowis inverted. Namely, in the embodiment of FIG. 13, the forward flowcancellation output signal VOUTF in which the component of thealternating current of the backward flow signal is added to the forwardflow signal, is obtained from the forward flow adjustment signal VOFadjusted in characteristic by using one cancelling means 90 a comprisingresistors 63, 64 and a capacitor 65. The backward flow cancellationoutput signal VOUTR in which the component of the alternating current ofthe forward flow signal is added to the backward flow signal, isobtained from the backward flow adjustment signal VOR adjusted incharacteristic by using the other cancelling means 50 a comprisingresistors 66, 67 and a capacitor 68. While the cancelling means 90 inthe present embodiment is constructed by one cancelling means 90 a andthe other cancelling means 90 b, it does not necessarily require bothcancelling means.

[0076] A switching means 91 comprising the signal comparing means orcomparater 69 and the signal selecting means or switching circuit 70,inputs the forward and backward flow adjustment signals VOF and VORadjusted in characteristic which are indicative of a forward and abackward direction of air flow, respectively, and controls the switchingoperation of the switch circuit 70. Namely, the direction of the forwardand backward flow is detected, the switching operation of the switchingcircuit 70 is performed, and the output signal VOUT (VOUTF or VOUTR) ofwhich the noise was cancelled is output. It is possible to use theforward flow detection signal V2F and the backward flow detection signalV2R, instead of the adjustment signals VOF and VOR, respectively.

[0077] As described above, it is possible to attenuate or cancel andeliminate the differential mode noise without large delay of detectionat the time when the air flow is inverted.

[0078] To sum up, in the present invention, the component of thealternating current of the backward flow detection signal is added tothe forward flow detection signal, and that of the forward flowdetection signal is added to the backward flow detection signal. Namely,the circuit is constructed so as to cancel the differential mode noisesincluded in two detection signals by adding the components of thealternating currents of the respective detection signals opposed to theforward flow detection signal V2F and the backward flow detection signalV2R to each other.

[0079] In other words, if an air flow meter which can detect a backwardflow is constructed by two air flow rate detecting parts each includinga heating resistor, the two heating resistors thermally interfere witheach other as they are thermally close to each other, the excessquantity of heat is transferred to the other heating resistor as aheating current supplied to one heating resistor increases, and theheating current is decreased by the air flow rate detecting part on theside of the heating resistor suffered from the excess quantity of heat.As a result, two heating current detection signals (detection signalsV2F, V2R) include the noise components of which the phases are inverted.Accordingly, the present invention is constructed so as to eliminate theabove noise components by adding the components of the alternatingcurrents of the two heating current detection signals to each other.

[0080] In the first embodiment, the cancelling means 90 further caninclude a signal selecting means for selecting either one of the forwardflow cancellation output signal and the backward flow cancellationoutput signal.

[0081]FIG. 14 is a circuit diagram showing a heating resistor type airflow rate measuring apparatus according to another embodiment related tothe second object of the present invention. The second embodiment willbe explained.

[0082] In the embodiment of FIG. 14, a differential amplifying circuit95 comprises a circuit having an operational amplifier 76, resistors 63,74, 75, 77 and a reference voltage VCM, which outputs a flow rate signalVG indicative of both the forward flow and the backward flow directions,in which the circuit is constructed so as to input the adjustment signalVOF or VOR selected by the switching circuit 70 switched by the outputof the comparater 69, and output a flow rate signal VG indicative ofboth directions of the forward and the backward flow and indicative ofthe reference voltage VCM when the flow rate is at zero (0), in additionto a circuit having a resistor 64 and a capacitor 65 for adding inadvance only the component of the alternating current of the backwardflow adjustment signal VOR.

[0083] Namely, in this embodiment, only the component of the alternatingcurrent of the backward flow adjustment signal VOR is added to theforward flow adjustment signal VOF by an alternating current signalpicking-up means having the resistor 64 and the capacitor 65, in orderto cancel the differential mode noises. In other words, the cancellingmeans, the switching means and the differential amplifier are formedintegrally in the apparatus according to the present embodiment.

[0084] According to the embodiment of FIG. 14, the differential modenoise of the forward flow adjustment signal VOF can be effectivelydecreased. Further, the differential mode noise of the backward flowadjustment signal VOR also can be decreased, which occurs when theheating resistor 41 for detecting the forward flow is thermallyconnected to the heating resistor 51 for detecting the backward flow,because the component of the alternating current within the range of thefrequency defined by the circuit having the resistor 64 and thecapacitor 65 which acts as a noise filter is cut off.

[0085] Further, if the output voltage of the apparatus shown in FIG. 14is set to the reference voltage VCM when the flow rate is at zero, it ispossible to operate the apparatus by a single power source for anautomobile.

[0086] In this embodiment, the delay of detection of the occurrence ofthe backward flow can be improved by correcting the original delay ofresponse in the heating resistor type air flow rate measuring apparatususing equalizers 72, 73 provided at the previous stage of the comparater69 for determining a direction of flow into which the forward andbackward flow adjustment signals adjusted in characteristic are input.It should be appreciated that it is possible to provide the equalizers72, 73 to other embodiments and to remove them.

[0087]FIG. 15 is a circuit diagram showing a heating resistor type airflow rate measuring apparatus according to a third embodiment of thepresent invention. The third embodiment will be explained with referenceto FIG. 15.

[0088] In the embodiment of FIG. 15, a differential amplifying circuit56 comprises a circuit having an operational amplifier 76, resistors 74,75, 77, 78 and a reference voltage VCM, in which the circuit isconstructed so as to input the adjustment signal VOF or VOR selected bythe switching circuit 70 switched by the output of the comparater 69,and output an output voltage indicative of the reference voltage VCMwhen the flow rate is at zero, and a circuit having a resistor 67 and acapacitor 68. In this embodiment, the phase of the component of thealternating current of the forward flow adjustment signal VOF isinverted by the circuit having the resistor 67 and the capacitor 68, andthen the component of the alternating current with the inverted phase isinput to the operational amplifier 76. The cancelling means, theswitching means and the differential amplifier are formed integrally inthe apparatus according to the present embodiment

[0089] According to the embodiment of FIG. 15, the differential modenoise of the forward flow adjustment signal VOF can be decreased, whichoccurs when the heating resistors are thermally connected to each other,because the component of the alternating current within the range of thefrequency defined by the circuit having the resistor 67 and thecapacitor 68 which acts as a noise filter is cut off. The differentialmode noise of the backward flow adjustment signal VOR also can bedecreased.

[0090]FIG. 16 is a circuit diagram showing a heating resistor type airflow rate measuring apparatus according to a fourth embodiment of thepresent invention. The fourth embodiment will be explained withreference to FIG. 16.

[0091] In FIG. 16, an air flow meter 80 or the heating resistor type airflow rate measuring apparatus is constructed by the air flow ratedetecting parts 48, 58 and the characteristic adjusting circuits 61, 62.The forward and the backward flow adjustment signals VOF, VOR aretransmitted from the air flow meter 80 to a fuel injection amountcontrol unit 81 for an internal combustion engine. The two adjustmentsignals VOF and VOR in the fourth embodiment are the same as the forwardand the backward flow detection signals in a broad sense.

[0092] A signal comparing means 82, alternating current extracting means83, 84, signal adding means 85, 86 and a signal selecting means 87 areprovided inside of the fuel injection amount control unit 81 of a fuelinjection amount control system for an internal combustion engine. Thefuel injection amount control unit 81 produces the output signal VOUTincluding a directional component, and transmit the output signal VOUTto a signal processing means 88 as the information necessary to controlthe fuel injection amount. The signal processing means 88 processes theoutput signal VOUT and makes the flow rate signal VG.

[0093] Accordingly, one cancelling means 90 a comprises the alternatingcurrent extracting means 84 and the signal adding means 85, and theother cancelling means 90 b comprises the alternating current extractingmeans 83 and the signal adding means 86. The switching means 91comprises the signal comparing means 82 and the signal selecting means87. Namely, in this embodiment, the switching means for selecting one ofinput signals or two adjustment signals and the cancelling means foradding the alternating current component of one input signal to theother input signal and adding that of the other input signal to oneinput signal and outputting the resultant signals, are provided in thefuel injection amount control unit 81 which is a semiconductorelectronic circuit.

[0094] According to the embodiment of FIG. 16, it is possible to reducethe cost of a fuel injection amount control system, because the signalcomparing means 82, the alternating current extracting means 83, 84, thesignal adding means 85, 86, the signal selecting means 87 and so on canbe integrated with the fuel injection amount control unit 81, and it ispossible to share the arithmetic unit for the fuel injection amountcontrol unit 81 and the above-mentioned means.

[0095]FIG. 17 is a circuit diagram showing a heating resistor type airflow rate measuring apparatus according to a fifth embodiment of thepresent invention. This embodiment is different from the embodiment ofFIG. 16 in that the heating resistor type air flow rate measuringapparatus is not installed integrally with the fuel injection amountcontrol unit 81.

[0096] A cancelling means 90 of FIG. 17 is the same as the circuithaving the alternating current extracting means 83, 84 and the signaladding means 85, 86 shown in FIG. 16. Further, A switching means of FIG.17 is the same as the circuit having the signal comparing means 82 andthe signal selecting means 87 shown in FIG. 16.

[0097] Further, in FIG. 17, the heating resistor type air flow ratemeasuring apparatus shown in FIG. 13 is shown in blocks. Namely, thealternating current extracting means 83 of FIG. 17 corresponds to thealternating current extracting means having the resistor 67 and thecapacitor 68 in FIG. 13. The alternating current extracting means 84 ofFIG. 17 corresponds to the alternating current extracting means havingthe resistor 64 and the capacitor 65 in FIG. 13. The signal adding means85 of FIG. 17 corresponds to the signal adding means having the resistor63 and the connection part in FIG. 13. The signal adding means 86 ofFIG. 17 corresponds to the signal adding means having the resistor 66and the connection part in FIG. 13. Still further, the signal comparingmeans 82 of FIG. 17 corresponds to the comparater 69 of FIG. 13, and thesignal selecting means 87 the switching circuit 70 of FIG. 13.

[0098]FIG. 18 is a circuit diagram showing a heating resistor type airflow rate measuring apparatus according to a sixth embodiment related tothe second object of the present invention. In the embodiments of FIGS.13 to 18, while the apparatus including the characteristic adjustingcircuits 61, 62, the differential amplifier 71 and so on was explainedas the heating resistor type air flow rate measuring apparatus or theair flow meter 80, it should be appreciated that the heating resistortype air flow rate measuring apparatus means the air flow rate detectingparts 48, 58 integrated with the cancelling means 90 in a narrow sense.

[0099] Namely, such a heating resistor type air flow rate apparatus hasa cancelling means for cancelling the differential mode noise componentsincluded in forward and backward flow detection signals by adding thealternating current component of the backward flow detection signal tothe forward flow detection signal and adding the alternating currentcomponent of the forward flow detection signal to the backward flowdetection signal, in addition to means for detecting the forward flowdetection signal V2F from the heating current necessary to heat theheating resistor for the forward flow installed in the air passage tothe predetermined temperature and means for detecting the backward flowdetection signal V2R from the heating current necessary to heat theheating resistor for the backward flow installed in the air passage tothe predetermined temperature.

[0100] In this case, the air flow rate signal VG can be obtained byusing the forward flow cancellation output signal VOUTF and the backwardflow cancellation output signal VOUTR which are output from thecancelling means.

[0101] In a heating resistor type air flow rate measuring apparatus orair flow meter comprising an air flow rate detecting part and acharacteristic adjusting circuit, it is also possible to provide thecancelling means 90 between the air flow rate detecting parts 48, 58 andthe characteristic adjusting circuits 61, 62.

What is claimed is:
 1. A heating resistor type air flow rate measuringapparatus for measuring an air flow rate with a heating resistorinstalled in an air passage, wherein a couple of heating resistors areinstalled in said air passage; an air flow rate signal is output bycorrecting a difference value between an output signal of one heatingresistor and an output signal of the other heating resistor onto areference output signal obtained by a heating resistor as reference. 2.A heating resistor type air flow rate measuring apparatus according toclaim 1, wherein said correction of an air flow rate signal is performedby multiplying a constant value or an variable constant determined inresponse to an air flow rate and a difference value between outputsignals of said two heating resistors.
 3. A heating resistor type airflow rate measuring apparatus according to claim 1, wherein said twoheating resistors are placed at closed positions where said two heatingresistors interferes thermally with respect to air flow.
 4. A heatingresistor type air flow rate measuring apparatus according to claim 1,wherein an output signal of a heating resistor placed at a upper streamof a air flow is used as said reference output signal of said heatingresistor as reference.
 5. A heating resistor type air flow ratemeasuring apparatus according to claim 1, wherein said two heatingresistors include two independent drive circuits.
 6. A heating resistortype air flow rate measuring apparatus according to claim 5, whereinoutput signals obtained by said two heating resistors are adjusted by acircuit so as to be identical to each other with respect to an air flowfrom a certain direction.
 7. A heating resistor type air flow ratemeasuring apparatus in either one of claims 1 to 6, wherein a heatingresistor, a driving circuit associated with said heating resistor, a subair passage in which said heating resistor is installed, and a connectorpart used as an interface to an outside are formed as a single module;and a penetration hole connecting between an inside part and an outsidepart of a main air passage part composite member contains at least saidsub air passage part in said major air passage.
 8. A heating resistortype air flow rate measuring apparatus having a couple of heatingresistors interfering to each other thermally with respect to an airflow, each heating resistor including an independent drive circuit,wherein an output signal of said each heating resistor is extracted bymeans that output signals obtained by said two heating resistors areadjusted by a circuit so as to be identical to each other with respectto an air flow running in a definite direction.
 9. A heating resistortype air flow rate measuring apparatus according to claim 8, wherein anintake air flow rate signal is obtained by correcting a difference valuebetween an output signal of one heating resistor and an output signal ofthe other heating resistor onto a reference output signal obtained by aheating resistor as reference.
 10. A heating resistor type air flow ratemeasuring apparatus in either of claims 1 to 6, wherein a heatergenerating heat independently on a air flow rate is inserted between acouple of heating resistors used for measuring an air flow rate; andsaid couple of heating resistors are placed at positions occupying anupper stream side and a down stream side with respect to an air flow.11. A heating resistor type air flow rate measuring apparatus in eitherof claims 1 to 10, wherein a filter is mounted between an output partfor an output signal and an input part for a air flow rate signal in acontrol unit for a fuel injection control.
 12. A heating resistor typeair flow rate measuring apparatus in either of claims 1 to 11, wherein afuel injection control is performed in responsive to an output signal.13. A heating resistor type air flow rate measuring apparatus in which aforward flow detection signal is detected from a heating currentnecessary to heat a forward flow heating resist or installed in an airpassage to the predetermined temperature, and a backward flow detectionsignal is detected from a heating current necessary to heat a backwardflow heating resistor installed in the air passage to the predeterminedtemperature, comprising; a cancelling means for cancelling adifferential mode noise including in each of the detection signals byadding the component of the alternating current of the backward flowdetection signal to the forward flow detection signal and adding thecomponent of the alternating current of the forward flow detectionsignal to the backward flow detection signal.
 14. A heating resistortype air flow rate measuring apparatus provided with a pair of air flowrate detecting parts for detecting heating currents necessary to heat aforward and a backward flow heating resistor installed in an air passageto the predetermined temperature, respectively, as a forward flowdetection signal and a backward flow detection signal, in order tooutput an air flow rate signal including a directional component of theair flow in the air passage by using each detection signal, furthercomprising: a cancelling means for cancelling differential mode noisesincluding in the forward and the backward detection signals by addingthe component of the alternating current of the backward flow detectionsignal to the forward flow detection signal and adding the component ofthe alternating current of the forward flow detection signal to thebackward flow detection signal, and outputting the forward and thebackward flow cancellation signals; wherein an air flow rate signal isoutput by using the forward and the backward flow cancellation signalsinstead of the forward and the backward flow detection signals.
 15. Aheating resistor type air flow rate measuring apparatus according toclaim 13 or 14, wherein said cancelling means includes a signalcomparing means for determining a direction of the air flow by thecomparison of large and small of the forward and the backward flowdetection signals, and a signal selecting means for selecting one of theforward and the backward flow cancellation output signals on the basisof the result of determination.
 16. A heating resistor type air flowrate measuring apparatus according to claim 15, wherein said cancellingmeans, said signal comparing means and said signal selecting means isconstructed so as to be included in a semiconductor electronic circuitof a fuel injection amount control unit for controlling the amount offuel injection to an internal combustion engine by using the air flowrate signal.
 17. A heating resistor type air flow rate measuringapparatus comprising: a pair of air flow rate detecting parts fordetecting heating currents necessary to heat a forward and a backwardflow heating resistor installed in an air passage to the predeterminedtemperature, respectively, as a forward flow detection signal and abackward flow detection signal, a signal comparing means for determiningthe direction of the air flow in the air passage by the comparison oflarge and small of the forward and the backward flow detection signals,a signal selecting means for selecting one of the forward and thebackward flow detection signals on the basis of the result ofdetermination, and a differential amplifying circuit for switching andinputting the forward and the backward flow detection signals, adding analternating current component of the backward flow detection signal tothe input forward flow detection signal, and switching and outputtingeither one of an output signal higher than a reference voltage inproportion to the added signal and an output signal lower than thereference voltage in proportion to the input backward flow detectionsignal; wherein an air flow rate signal including a directionalcomponent of the air flow is output by using the output signal from thedifferential amplifying circuit.
 18. A heating resistor type air flowrate measuring apparatus comprising: a pair of air flow rate detectingparts for detecting heating currents necessary to heat a forward and abackward flow heating resistor installed in an air passage to thepredetermined temperature, respectively, as a forward flow detectionsignal and a backward flow detection signal, a signal comparing meansfor determining the direction of the air flow in the air passage by thecomparison of large and small of the forward and the backward flowdetection signals, a signal selecting means for selecting one of theforward and the backward flow detection signals on the basis is of theresult of determination, and a differential amplifying circuit forswitching and inputting the forward and the backward flow detectionsignals, inverting the phase of an alternating current component of theforward flow detection signal and adding the resultant signal to theinput backward flow detection signal, and switching and outputtingeither one of an output signal higher than a reference voltage inproportion to the forward flow detection signal and an output signallower than the reference voltage in proportion to the added signal;wherein an air flow rate signal including a directional component of theair flow is output by using the output signal from the differentialamplifying circuit.
 19. A fuel injection amount control system in whichthe amount of fuel injection to an internal combustion engine iscontrolled by using the air flow signal obtained by the heating resistortype air flow rate measuring apparatus according to either one of claims13 to 18.